Signal receiver circuit



Dec. 6, 1938. G. F. BRETT ET AL I SIGNAL'RECEIVER CIRCUIT Filed Aug. 19, 1936 M 601V M005 VOLTAGE Imam-0R5 GEORGE F. BRETT AND BY GEOR WRIGHT ATTORNEY Patented Dec. 6, 1938 UNITED STATES PATENT OFFICE SIGNAL RECEIVER CIRCUIT Delaware Application August 19, 1936, Serial No. 96,790 In Great Britain August 31, 1935 8 Claims.

This invention relates to thermionic valves, and to circuit arrangements incorporating the same, and more particularly to thermionic valves of the electron beam type and. to circuit arrange- 5 ments incorporating the same. By a valve of the electron beam type is meant a valve of the type wherein the discharge is of jet, or beam, form, and partakes more of the nature of that in a cathode ray tube than of that in an ordinary thermionic valve as at present in common use. Electron beam valves are described in British patent specification No. 403,973. Electron beam valves as described in the said specification No. 403,973 present a substantially fiat output anode current (ordinates)-gun anode-voltage (abscissae) characteristic curve.

The main object of the present invention is to provide improved electron beam valves wherein (with constant applied anode potential) the anode or collector current increases with increase of accelerator potential up to a predetermined value, but, when this value is exceeded the said anode or collector current does not further increase but falls somewhat; i. e., to provide im proved electron beam valves exhibiting negative resistance efiects.

Another object of the invention is to provide circuit arrangements embodying electron beam valves exhibiting a characteristic as just set forth, and wherein said characteristic is taken'advantage of for automatic gain control purposes.

According to this invention an electron beam valve comprises an electron gun system an anode electrode being positioned in the normal path of the electrons from the said electron gun system, and an additional electrode situated on the side of the anode electroderemote from the gun, said additional electrode and anode electrodes being so arranged and disposed relative to one another that by varying the cross sectional area of the beam at the anode electrode the proportion between the electron currents passing to said anode electrode and said additional electrode respectively will be varied.

The specification accompanying co-pending British application No. 34,885/35 describes eleotron beam valves exhibiting negative resistance eiiects, the invention contained in the said copending specification consisting in providing an electron beam valve (in addition to the anode and the electron gun system for projecting a beam of electrons towards the anode and for controlling the intensity of the said beam) with a collector electrode on the side of the anode remote from the gun the said anode being so slotted or otherwise shaped and positioned that the amount of cross section of the beam intercepted by the said anode may be varied by varying the cross section of said beam at the anode any portion of the beam not intercepted by the anode falling 5 on the collector electrode. No claim is made in this specification to such a valve per se, and the claims of the'present specification-are intended to be read subject to this limitation.

In a preferred form of valve in accordance 10 with this invention the additional electrode is of larger projected area (when viewed from the gun) than the said anode electrode, the whole arrangement being such that when the electron beam or jet from the gun does not exceed a pre- 15 determined spread or cross sectional area at the anode electrode, most of the electrons are collected by the said electrode, but when the spread or cross sectional area of the said jet or beam is increased beyond this value a larger propor- 20 tion of the electrons miss the anode electrode and fall upon the additional electrode. The spread or cross sectional area of the beam will, of course, depend inter alia upon the potential applied to the accelerator electrode or electron gun anode, 5 increasing with increase of such potential in the positive direction.

An important application of electron beam valves exhibiting negative resistance effects is to automatic gain control circuits. It is, of course, 0 very common in electron discharge device am plifiers to provide means for automatically controlling the gain or sensitivity in dependence upon incoming signal strength. One of the commonest and most important cases of such provision 35 is that of broadcast radio receivers Where, in order to militate against the effects of fading, and to enable the reproduced volume, when receiving from a weak station, to be about the same as that when receiving from a powerful station while 40 avoiding overloading of the receiver in the latter event, it is usual to rectify the amplified received carrier and utilize the rectified potential as gain controlling grid bias upon one or more carrier frequency so-called variable-mu valve stages 5 in the receiver.

The property possessed by electron beam valves exhibiting negative resistance effects may be utilized, in applying the present invention to enable automatic gain or volume control to be obtained in a very simple and satisfactory manner, and, as will be seen later, such valves can be used to great advantage in radio receivers and other electron beam valve apparatus where automatic gain or volume control is required. As will now be appreciated with a valve in accordance with the present invention, or as described in the aforesaid British co-pending specification No. 34,885/35, if the voltage applied to the anode of the electron gun of the valve be increasedi. e. if the potential of the electron accelerating electrode be increased-Without changing any of the other normal operating potentials, the magnification of the valve increases to a fairly well defined maximum and thereafter decreases, the curve of decrease being very similar to that of the curve of increase. In fact, if a curve connecting magnification (ordinates) of such an electron beam valve with gun anode voltage (ab scissae) be drawn the result is as examplified by the accompanying Figure 1; the said curve having a rounded maximum and being substantially symmetrical about a vertical line AM passing through the maximum, the rising part of the curve consisting of an approximately rectilinear sloping portion from X to C which merges into a curved portion CA whose slope falls away increasingly quickly as the curve rises until the said curve becomes horizontal at the maximum A. It will, therefore, be seen that the gain of an electron beam Valve of the nature in question may be varied by varying the gun anode potential, the shape of the characteristic curve being such as to enable excellent gain control effects to be obtained.

According to a feature of this invention automatic gain or Volume control is obtained in an electron discharge device circuit arrangement comprising one or more electron beam valves as hereinbefore set forth by deriving a uni-directional potential or value dependent upon signal strength and utilizing said potential to control the gun anode or accelerating anode potentials of at least one of said electron beam valves.

In the drawing:

Fig. 1 shows an operating characteristic of a tube embodying the invention,

Fig. 2 is a schematic representation of a tube employing the invention,

Fig. 3 shows a receiver circuit employing the invention.

Referring to Figure 2 this shows a valve in accordance with. the invention. having an electron gun system consisting of a rectilinear cathode I (which may be directly or indirectly heated) coaxially constituted within a cylindrical control electrode or grid 2 having a split, or slit, 2a running parallel to the cathode axis. The gun system also includes an accelerator electrode 3 in the form of a plate also provided with a slit or split 3a and a suppressor or screening electrode 4 which is also provided with a slit or split 4a. The cathode, the grid, the accelerator electrode, and the suppressor electrode are mounted close together in the order stated from the cathode, and the slits or splits in the three cold electrodes of the said gun system are so positioned and dimensioned that a ribbon-like jet or beam of electrons will (when. the valve is in use) pass from the cathode and proceed through the successive slits or splits. (Figure 2 is a transverse schematic section, the lengths of the split or slitswhich are rectangular or approximately sobeing at right angles to the plane of the paper.)

In the normal path of this jet or beam of electrons is an anode electrode 5 which is preferably of U-section (though it may be a narrow flat plate) the U facing towards the gun system. Behind the anode 5 is an additional electrode 6 of plate form, and of a projected area (as viewed from the cathode) considerably in excess of that of the anode electrode. Typical voltages which may be applied in use to a valve as just described are: zero voltage upon the cathode I; a small negative bias voltage upon the. grid 2; a variable voltagewhich may vary between say 20 volts and +150-upon the accelerator electrode 3; zero voltage upon the suppressor electrode 4 (this may be connected either internally or externally of the valve envelope to the cathode); 150 volts upon the anode electrode 5; and 250 volts upon the additional electrode 6.

Regarding the accelerator electrode 3 as a variable voltage electrode it will be appreciated that when a low voltage is applied to the said accelerator electrode, the jet or beam of electrons will be narrow and of small total current; and most, if not all, of the electrons will fall upon the anode electrode 5. For increasing accelerating voltages the beam current will increase, but a large proportion of the electrons will still fall on the anode electrode 5 up to a certain value. As the accelerator electrode voltage increases, however, the beam becomes more and more divergent so that a point is reached when a further increase in potential, though still causing the total current to increase, will also cause the proportion of electrons reaching the anode electrode 5 to decrease, more electrons reaching the additional electrode 6. When the accelerator electrode voltage becomes high enough the beam will be so divergent that very little of it will fall on the anode electrode, but most of it will be incident upon the additional electrode 6. It will be seen, therefore, that the curve connecting accelerator voltage and anode electrode current will show a maximum. This fact may be utilized for many control purposes-notably for automatic volume control.

The advantage of making the anode electrode of U-section is that it prevents undesired loss of electrons to the additional electrode, which latter is maintained at a high voltage so that it may fully serve its purpose. It is desirable to arrange and construct the additional electrode with a view to the minimizing of secondary emission therefrom, so that the said additional electrode shall not lose, as a result of secondary emission to the anode electrode, electron current which is incident upon it.

Figure 3 shows part of a simple radio receiver including a tuned high frequency electron beam valve stage comprising a valve VI, as shown in Figure 2, followed by a demodulating (ordinary valve) anode bend detector stage comprising a triode V2. Signals to be amplified are applied, as in the usual way, to a circuit 10 extending between the control electrode 2 of the gun of the electron beam valve VI and the cathode I thereof, amplified signals being taken from the anode 5 of said valve and applied to the grid circuit of the demodulating detector V2 which is foilowed in cascade by any desired suitable audio frequency apparatus (not shown). The gun anode 3 of the electron beam valve VI is not operated at a fixed potential, but is caused to vary in potential in dependence upon the rectified unidirectional output from the detector V2. In the simplest case this result is achieved as shown by a connection between the detector anode AV2 and the gun anode 3. This connection may include a suitable delay or smoothing circuit (not shown) as well known per se to prevent Variation of the gun anode potential at modulating (audio) frequencies. Obviously other forms of detector may be used.

If with the arrangement of Figure 3 the circuit elements be so chosen and adjusted that normally (i. e. in the absence of signals) the potential on the gun anode (3) is such as to cause the electron beam valve to work at the maximumampliflcation point A of the curve of Figure 1, the arrival of a strong carrier will cause reduction of amplification so that sensitivity or gain will be kept high for very weak or zero signal strength but will decrease with increasing signal strength.

An alternative adjustment is that wherein the normal gun anode potential is such as to bring the operating point on the downwardly sloping part of the curve, below the maximum e. g. at the point B in Figure 1. With this setting the sensitivity or gain will be below maximum for zero signal strength but will rapidly rise with increasing signal strength until, when the maximum point on the curve is reached, further increase in signal strength causes reduction in gain or sensitivity. This adjustment has the advantage that since the sensiitivity is below'maximum for very weak signals reproduced strength corresponding to stray interference (mush") picked up when tuning the receiver between stations will be reduced but when the receiver is accurately tuned to a desired station which, though weak, is strong enough to be received reasonably, the sensitivity will increase. When a strong station is received the sensitivity will be decreased to prevent overloading and preserve an approximate constancy of reproduced strength.

Since the characteristic curve is substantially symmetrical about a vertical line through the maximum, and, over part of the curve, an increase in gun anode voltage produces a decrease in magnification, a stage of any known type of direct current amplification e. g. a resistance coupled amplifier may be included between the control point of the receiver (in the circuit of Figure 3 the detector anode AV2) and the controlled point, which is the gun anode 3. Such a resistance coupled amplifier will, of course, reverse the phase of gun anode potential with signal intensity, but the desired gain control effect is, nevertheless, still obtainable owing to the shape of the curve. The advantage of including such a direct current amplifier is that a smaller change in signal strength is necessary to produce a given degree of control; in other words the sensitivity of the gain control action is increased and therefore a closer approximation to constant reproduced strength, despite fluctuations in incoming strength, is obtainable. Where a single stage direct current amplifier is provided as above described, or a direct current amplifier having an odd number of stages, the operating point on the characteristic curve may with advantage be selected below the maximum on the rising portion e. g. at the point C of Figure 1, so as to obtain the advantages set forth in the last preceding paragraph. The same effect will result if a grid leak detector or the now common diode-triode type of detector stage be substituted for an anode bend detector.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed we declare that, subject to the limitation hereinbefore set forth, what we claim is:

1. In a wave transmission system, a beam tube provided with an electron emitter, a control electrode, an output electrode of substantial concavity facing the cathode, a plate of relatively wide area to the rear of the output electrode and an acceleration electrode disposed between the cathode and output electrode, said acceleration electrode being provided with an opening through which passes the electron beam from the emitter to said output electrode, a wave input circuit coupled to said control electrode, a wave output circuit coupled to the output electrode, means for maintaining said plate and output electrode positive relative to said cathode, and means for varying the potential of said acceleration electrode thereby to control the cross-sectional area of the beam at said output electrode and thus the electrons collected by said plate.

In a wave transmission system, a beam tube provided with an electron emitter, a control electrode, an output electrode of substantial concavity facing the cathode, a plate of relatively wide area to the rear of the output electrode and an acceleration electrode disposed between the cathode and output electrode, said acceleration electrode being provided with an opening through which passes the electron beam from the emitter to said output electrode, a wave input circuit coupled to said control electrode, a wave output circuit coupled to the output electrode, means for maintaining said plate and output electrode positive relative to said cathode, and means for varying the potential of said acceleration electrode thereby to control the cross-sectional area of the beam at said output electrode and thus the electrons collected by said plate, said last means being responsive to the wave amplitude. In a wave transmission system, a beam tube provided with an electron emitter, a control electrode, an output electrode of substantial concavity facing the cathode, a plate of relatively wide area to the rear of the output electrode and an acceleration electrode disposed between the cathode and output electrode, said acceleration electrode being provided with an opening through which passes the electron beam from the emitter to said output electrode, a wave input circuit coupled to said control electrode, a wave output circuit coupled to the output electrode, means for maintaining said plate and output electrode positive relative to said cathode, means for varying the potential of said acceleration electrode thereby to control the cross-sectional area of the beam at said output electrode and thus the electrons collected by said plate, said last means comprising a wave rectifier coupled to said output circuit, and a direct current voltage connection between the rectifier and the acceleration electrode.

4. In a wave transmission system, a beam tube provided with an electron emitter, a control electrode, an output electrode of substantial concavity facing the cathode, a plate of relatively wide area to the rear of the output electrode and an acceleration electrode disposed between the cathode and output electrode, said acceleration electrode being provided with an opening through which passes the electron beam from the emitter to said output electrode, a wave input circuit coupled to said control electrode, a wave output circuit coupled to the output electrode, means for maintaining said plate and output electrode positive relative to said cathode, and means for varying the potential of said acceleration electrode thereby to control the cross-sectional area of the beam at said output electrode and thus the electrons collected by said plate, the said emitter being in alignment with said concavity and opening.

5. In combination with an electron beam tube of the type including an emission element, an output electrode arranged to receive an electron beam from the emission element, and a control electrode, means for impressing signals upon the control electrode of the tube, an output circuit connected to the output electrode of the tube, a signal rectifier connected to be responsive in direct current voltage output to the signal amplitude, an auxiliary electrode disposed within said tube to vary the number of electrons reaching said output electrode, and a direct current voltage connection from said auxiliary electrode to the signal rectifier such that the number of electrons of said beam reaching said output electrode is decreased as the signal amplitude increases.

6. In combination with an electron beam tube of the type including an emission element, an output electrode arranged to receive an electron beam from the emission element, and a control electrode, means for impressing signals upon the control electrode of the tube, an output circuit connected to said output electrode, a signal rectifier connected to be responsive in direct current voltage output to the signal amplitude, an auxiliary electrode disposed within said tube to vary the number of electrons reaching said output electrode, and a direct current voltage connection from said auxiliary electrode to the signal rectifier such that the number of electrons of said beam reaching said output electrode is decreased as the signal amplitude increases, said auxiliary electrode comprising a plate provided with an opening through which the beam passes, and said last named plate being disposed between the output electrode and said emission element.

7. In combination with an electron beam tube of the type includingan emission element, an

output electrode arranged to receive an electron beam from the emission element, and a control electrode, means for impressing signals upon the control electrode of the tube, an output circuit connected to the output electrode of the tube, a signal rectifier connected to be responsive in direct current voltage output to the signal amplitude, an auxiliary electrode disposed within said tube to vary the number of electrons reaching said output electrode, a direct current voltage connection from said auxiliary electrode to the signal rectifier such that the number of electrons of said beam reaching said output electrode is decreased as the signal amplitude increases, and a plate disposed behind said output electrode and having a relatively large surface extending, said beam electrons falling on said plate in increasing numbers as said auxiliary electrode decreases the number of electrons flowing to said output elec trode.

8. In combination, in a radio receiver, a signal input circuit, a signal output circuit, an amplifier tube of the type including an emission element, an output electrode disposed within an amplifier tube to receive electrons in the form of a beam from the emission elements, a control electrode in said tube and being connected to said signal input circuit, said output electrode being connected to said output circuit, a plate of relatively large area disposed adjacent said output electrode, an electrode disposed between said output electrode and said emission element and being constructed to vary the cross-sectional area of the beam to said output electrode, a signal rectifier, and a direct current voltage connection between the signal rectifier and said beam area control electrode.

GEORGE FAIRBURN BRETT. GEORGE MAURICE WRIGHT. 

